This invention relates to the manufacture of ferrous alloys by a process including an oxygen refining step. The oxygen refining step may typically comprise decarburisation but may alternatively or additionally comprise removal of silicon or manganese.
Intermediate carbon ferrochrome is made commercially by the partial oxidation of the carbon content of so called “charge chrome”, an alloy of iron and chromium containing a relatively high proportion of carbon (typically in the order of 6 percent by weight). (Ferrochrome is another name for ferrochromium.) The partial oxidation is effected in a converter by blowing a mixture of oxygen and steam through the molten alloy by means of one or more submerged tuyeres. A ferrochrome product containing less than 2% by weight of carbon can be produced. A slag is formed during the oxidation that can contain a substantial amount of chromium oxide. The chromium oxide is typically recovered by the addition of a ferrosilicon reductant at the end of the process cycle. Nevertheless, some chromium oxide is lost in the slag that is formed in this primary reduction step.
A reduced carbon ferromanganese can be made commercially by an analogous process to that described above for the manufacture of ferrochrome.
Stainless steel is a low carbon ferrous alloy typically including chromium and nickel as alloying elements. A typical composition contains 18% by weight of chromium, 8% by weight of nickel, less than 0.1% by weight of carbon, the balance being iron and any other alloying elements (excluding incidental impurities). Stainless steel is typically made by melting a charge of stainless steel scrap and high carbon ferroalloys in an electric arc furnace to form a crude alloy containing up to 0.5% by weight more chromium than is desired in the product and having a carbon content in the range of 0.25% to 2% by weight and a silicon content in the range of 0.2% to 1.5% by weight. The particular levels of carbon and silicon depend on the product specification, steel making practice and vessel size. The crude alloy is transferred in molten state to a converter in which the molten alloy is blown from beneath the surface with oxygen so as to oxidise the carbon to carbon monoxide and thus decrease the carbon content of the resultant stainless steel to less than 0.1% by weight. As the carbon level progressively decreases during the blow, so there is a tendency for the oxygen to react with the chromium to form chromium oxide. There is also an associated tendency for an excessive temperature to be created in the converter because of the exothermic nature of the oxidation reactions. In the Argon-Oxygen Decarburisation (AOD) process this tendency is counteracted by progressively, or in steps, diluting the oxygen with argon so as to reduce the partial pressure of carbon monoxide and so promote carbon oxidation in preference to oxidation of chromium. By these means most of the chromium is retained in the bath and temperature increases can be restricted to an acceptable level (for example to a temperature no higher than 1750° C.). In a typical example, the blow is commenced with an argon-oxygen ratio (by volume) of 1:3 and may finish with an argon-oxygen ratio (by volume) of 2:1. After the blow, some ferrosilicon can be added to reduce chromium oxide in the slag, and lime can be introduced as a desulphurisation agent.
The Creusot-Loire-Uddeholm (CLU) process may be used as an alternative to the AOD process. The CLU process is analogous to the AOD process but typically uses a mixture of steam, nitrogen and argon instead of pure argon to dilute the oxygen that is blown into the melt from beneath its surface.
All the processes mentioned above have in common the refining with oxygen of a ferroalloy having an appreciable carbon content in order to reduce its carbon content. Even with the dilution of the oxygen with, for example, argon, these processes still exhibit a tendency towards progressive damage of the refractory lining of the converter particularly in the vicinity of each tuyere through which the oxygen is blown. Regular relining of the converter is therefore necessary.
U.S. Pat. No. 4,434,005 discloses a method for refining a molten metal overlain by a slag and which cold solids are introduced, for example in the form of metal scrap. The heat necessary to melt the scrap and prevent undue cooling of the bath is generated by directing a jet of neutral gas entraining carbon against the surface of the melt at a supersonic velocity while oxygen for refining purposes is directed at the surface from separate and non-shrouding jets and the metal is bottom blown by neutral gas to prevent excessive foaming of the slag.
JP-A-61284512 discloses the production of high chrome steel by mixing chrome ore and coke powders in a blowing nozzle and blowing the mixture into the firing point of the molten iron both to melt and reduce the chrome ore.
GB-A-2 054 655, GB-A-2 122 649, and JP-A-58207313 relate to basic oxygen steel making processes in which molten metal is top-blown with oxygen and bottom blown with a different gas. Solids may be introduced with the gases.
JP-A-61106744A relates to the introduction of oxygen and solids into a furnace through tuyeres during the manufacture of stainless steel.